Exploring the inner region of type 1 AGNs with the Keck interferometer

The exploration of extragalactic objects with long-baseline interferometers in the near-infrared has been very limited. Here we report successful observations with the Keck interferometer at K -band (2.2  μ m) for four type 1 AGNs, namely NGC 4151, Mrk231, NGC 4051, and the QSO IRAS 13349+2438 at $z = 0.108$. For the latter three objects, these are the first long-baseline interferometric measurements in the infrared. We detect high visibilities ( V 2 ~ 0.8–0.9) for all the four objects including NGC 4151, for which we confirm the high V 2  level measured by Swain et al. (2003, ApJ, 596, L163). We marginally detect a decrease of  V 2 with increasing baseline lengths for NGC 4151, although over a very limited range, where the decrease and absolute  V 2 are well fitted with a ring model having a radius of 0.45  ±  0.04 mas (0.039  ±  0.003 pc). Strikingly, this matches independent radius measurements from optical-infrared reverberations that are thought to be probing the dust sublimation radius. We also show that the effective radius of the other objects, obtained from the same ring model, is either roughly equal to or slightly larger than the reverberation radius as a function of AGN luminosity. This suggests that we are indeed partially resolving the dust sublimation region. The ratio of the effective ring radius to the reverberation radius might also give us an approximate probe for the radial structure of the inner accreting material in each object. This should be scrutinized with further observations.